Edit: We filled up a bottle full of the GreatStuff foam, and when it cured completely, we could squish it and it would form back to its shape. I see no reason not to atleast try using it. It will still after all impart some of the forces contacting the piston's rear and still serve its purpose as a bumper just fine.

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Spudding since '05. Proud waster of plumbing and plumbing accessories.